I have a door and a timer switch. If the door and the switch are both in the same room, it works as expected (ie. door opens and then closes after 12s).
But if I move the switch into another room, it stops working (door doesn't open).

If I use normal switch (no timer), door doesn't open either. BUT if I add another trigger on the switch trigger (ie CD or Flyby), it all works (door opens, music / flyby plays). But for the timer switch it doesn't work.

My ideal setup would be timer switch, door opening, music playing.
Any clue how to fix this? How is it even possible?

My setup is SWITCH FOR SWITCHTYPE (Timer space with number 12), TRIGGER FOR DOORTYPE. I tried opening some other door in another room, not working either (different DOOR slot).
Is there some limitations as what DOOR type and SWITCH type can be used in timer switch?

Is the timer switch a different object from the other switch that works? Or are you using the same object and it's the trigger setup that differs? If they are different objects, compare their setups, there must be something different there. I am not sure if there is a restriction in terms of the object slots, except that I think SWITCH_TYPE7 and SWITCH_TYPE8 are only for shootable switches.

If you are saying that all you have to do is add a number to the "timer" field to get it to stop working...that would be bizarre. I could only guess you have some kind of special NGLE trigger that is also using a timer field, maybe...

The timer switch is the same object as the other switch without timer. So it's the same object, just the trigger setup differs.
It's SWITCH_TYPE1 and DOOR_TYPE3.

The thing is, even normal SWITCH trigger doesn't work if the switch item and door item are both in different rooms. But if I put them in the same room, it works.

I did a workaround, created a big room under those rooms I used before and moved the door and switch into the one big room downstairs. Above this room are 3 different rooms (2 of them contained the switch and the door).
So, the switch and the door are both in the same room and it works, even with timer switch...

But still, it's weird, it's supposed to work in different rooms as well.

Nope. All movables may contain some kind of information about sounds, state-ID's, next animation and so on. Only statics (starting from plant0) are perfectly safe. And if some of that information has been changed (often by accident), objects may cause bugs.

So when you one day (happens to every builder) experience, that sounds in your game have been messed up (wrong sounds playing or sounds missing completely), you should take a look at your wad and remove the latest added object(s) to see, if that helps.